Embodiments of the present invention are directed to power supply control circuits and power supply systems. More particularly, embodiments of the present invention provides methods and circuits for controlling a power transistor in a switching mode power supply (SMPS).
Regulated power supplies are indispensable in modern electronics. For example, the power supply in a personal computer often needs to receive power input from various outlets. Desktop and laptop computers often have regulated power supplies on the motherboard to supply power to the CPU, memories, and periphery circuitry. Regulated power supplies are also used in a wide variety of applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
In general, a power supply can be regulated using a linear regulator or a switching mode controller. A linear regulator maintains the desired output voltage by dissipating excess power. In contrast, a switching mode controller rapidly switches a power transistor on and off with a variable duty cycle or variable frequency and provides an average output that is the desired output voltage.
Compared with linear regulators, switching mode power supplies have the advantages of smaller size, higher efficiency and larger output power capability. On the other hand, they also have the disadvantages of greater noise, especially Electromagnetic Interference at the power transistor's switching frequency or its harmonics.
Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) are two control architectures of switching mode power supplies. In recent years, green power supplies are emphasized, which require higher conversion efficiency and lower standby power consumption. In a PWM controlled switching mode power supply, the system can be forced to enter into burst mode in standby conditions to reduce power consumption. In a PFM controlled switching mode power supply, the switching frequency can be reduced in light load conditions. PFM-controlled switching mode power supply exhibits simple control topology and small quiescent current. Therefore, it is suitable for low cost small output power applications such as battery chargers and adapters.
In such a switched mode power supply system, a switch is connected to the primary winding of the transformer. In the switching power supplies, the power transistor switch on and off periodically to convert the primary current of the transformer to the secondary side. The stable output voltage will be obtained by regulating the duty cycle or frequency of the primary side switch. Magnetic energy is stored in the inductance of the primary winding when the switch is turned on, and the energy is transferred to the secondary winding when the switch is turned off. The energy transfer results in a current flowing through the secondary winding and the rectifying diode. When the energy transfer is completed, i.e., the current stops flowing through the diode, a substantially sinusoidal oscillation of decreasing amplitude appears at the secondary winding.
The frequency of the sinusoidal oscillation is determined, in part, by the inductance of the primary winding and by the parasitic capacitance in the primary winding as well in the printed circuit board. The effect of these components often are difficulty to determine in advance and can lead to output performance limitations in the switched mode power supply. Some of these limitations are described in more detail below.
Therefore, there is a need for techniques that can provide more effective control of the power transistor in a switching mode power supply.